This invention relates to a handoff method of a cellular communication system, a personal communication system and a satellite communication system using a Code Division Multiple Access (CDMA) system, more particularly to a handoff trial method by a mobile station that compares a pilot power strength of a base station providing communication service to the mobile station with a pilot power strength of a neighboring base station and tries handoff based on the comparing results.
In a mobile communication system, service area of a mobile station may be changed according to moving of the mobile station. The service area, a cell may be classified according to a pilot offset and a pilot power strength to be outputted through a pilot channel, in which the pilot offset and the pilot power strength differentiate the pilot channel. Therefore whether a mobile station situated at a cell border region crossing two cell belongs to any one out of the two base station service area may be recognized by detecting a received pilot power strength having a pilot offset of the mobile station transmitted to associated base station. Also, even if the mobile station moves in inter-cell, as it is necessary to provide communication service to the mobile station without corruption, handoff that automatically changes over communication line in inter-cell moving of the mobile station to sequentially provide communication service must be performed.
Referring to FIG. 1, there is shown a border diagram of handoff between two base stations in a general mobile communication system.
In the drawing, the reference number 10 designates a mobile station for receiving communication service while moving, the reference number 20 designates a base station for providing communication service to the mobile station 20, reference number 25 designates other base station neighboring the base station 20 and reference number 30 designates a border that is in the cell (right oval) of the base station 20 while the cell (left oval) of the base station 25 originates. In addition, the reference number 35 designates border that is in the cell (left oval) of the base station 25 while the cell (right oval) of the base station 20 originates and the reference number 40 designates border that the base station 20 has the same pilot power strength as the base station 25.
In FIG. 2 there is illustrated a flowchart for general method that the mobile station 10 tries handoff in situation of FIG. 1. Handoff procedure of the prior art of FIG. 2 will be explained in conjugation with the FIG. 1
First of all, when the mobile station 10 has been in the cell of the base station 20 that provides communication service to the mobile station 10 and is gradually moving to the border 30, the mobile station 10 measures and stores a pilot power strength (pilot_A) of the base station 20 and a pilot power strength (pilot_B) of the base station 25(ST1-ST2). Then the mobile station 10 compares the pilot_B with pilot threshold_0 that is an absolute value set itself to try handoff (ST3). With the result that, if the pilot_B is larger than the pilot threshold_0, the mobile station 10 transmits a signal measurement message having the pilot offset and power strength of the pilot_B to the service base station 20 to try handoff to the base station 25(ST4). The signal measurement message is transmitted in sequence to the base station 20 until the mobile station 10 moves in sequence to the cell region of the base station 25 and the handoff will have completed. Otherwise, if the pilot_B is smaller than the pilot threshold_0, flow returns to the ST1 in FIG. 2.
Meanwhile, when the mobile station 10 has been in the cell of the base station 25, which provides communication service to the mobile station 10, and is gradually moving to the border 35, the mobile station 10 measures and stores the pilot power strength of the base station 20. If the measured pilot power strength of the base station 20 is larger than a pilot threshold set to the mobile station 10 to try handoff, the mobile station 10 transmits a signal measurement message having the pilot power strength of the service base station 25 to try handoff to the base station 20. As described in the above, each handoff region of the base stations 20 and 25 is between the border 30 and border 35.
Accordingly, it is noted in the handoff method of the prior art that the pilot power strength of neighboring base station is only inspected without inspecting the pilot power strength of service base station. If the pilot power strength of neighboring base station is larger than the pilot threshold set to the mobile station to try handoff, the mobile station unconditionally sends the signal measurement message for handoff to the base station. Although handoff isn""t needed as the pilot power strength of the service base station is enough to provide communication service, handoff will be tried if the above condition is satisfied.
Therefore, the handoff trial method of the mobile station in the prior art have to send the signal measurement message to the service base station at broad inter-cell region such as region between the border 30 and border 35 in FIG. 1, which causing to increase power consumption to handoff. Further, as handoff region is broad, the prior art has the problem in that the base station has much loads to process the signal measurement message received from the mobile station and use many channels due to frequent handoff.
U.S. Pat. No. 5,737,704 discloses channel allocation method for reducing inter-cell hard handoffs in a CDMA system which divides a service area into a plurality of cells and allocates a channel to each cell. The ""704 patent comprises a first step of dividing a cell into a plurality of concentric circle regions, a second step for allocating a first channel not allocated to a neighboring cell, a third step of allocating a second channel allocated to the neighboring cell to the outermost concentric circle region of the cell and a forth step of allocating the first and second channels allocated by the second step and third step to a middle concentric circle region of the cell region. Also, the first and second region is selectively varied in order to reduce handoff.
The U.S. Patent shows a method that divides a service area into several cells by Round Trip Delay and allocates one channel to each cell, in which a cell is divided into 3 parts of an innermost concentric circle region, a middle concentric circle region and an outermost concentric circle region, channels of innermost and outermost regions being allocated to the middle concentric circle region, so as to reduce handoff by means of cell dividing method. The method, however, disadvantages in that has complex procedures such as dividing one cell into several regions and measuring RTD several times.
Again referring to FIG. 1, standing at the border 40 of forward handoff, the mobile station 10 receives same pilot power strengths from the base stations 20 and 25, but receives pilot power strength larger by a predetermined value from the base station 20 at the cell border 30. Thus, comparing the pilot power strength of service base station 20 with that of the neighboring base station 25, we could find where the mobile station 10 stands. This inventor pays attention to the above point to solve the prior art problem.
It is therefore an object of the present invention to provide a handoff trial method by a mobile station which measures and stores a pilot power strength of a base station providing communication service to the mobile station, measures a pilot power strength of a neighboring base station and compares the stored pilot power strength of the service base station with the measured pilot power strength of the neighboring base station to try handoff according to the comparing result and to reduce occurrence frequency of handoff, and thereby to reduce power consumption of the mobile station and signal processing loads of the service base station.
According to a first aspect of the invention, the above object is achieved by a handoff trial method by a mobile station comprising the steps of: a first step of measuring and storing a pilot power strength (pilot_A) of a base station on service; a second step of measuring and storing a pilot power strength (pilot_B) of a neighboring base station; a third step of comparing the sum of the pilot_A and pilot threshold_1 set to the mobile station to try handoff with the pilot_B; a forth step of, if the pilot_B is larger than the sum of the pilot_A and pilot threshold_1 as a result of the comparing, storing associated pilot offsets out of the pilot_B at predetermined times and counting the stored times; a fifth step of identifying whether the counting times reaches at a predetermined times to detect a pilot signal suited for handoff condition; a sixth step of, if the counting times reaches at the predetermined times as a result of the identifying, identifying whether the stored pilot offsets are all the same; and a seventh step of, if the pilot offsets are all the same as a result of the identifying, sending a signal measurement message having the stored pilot offsets and associated pilot power strengths to the service base station and requesting handoff therefrom.
Other object of the present invention is to provide a handoff trial method by a mobile station which measures and stores pilot power strengths of more than two base stations providing communication service to the mobile station, measures pilot power strengths of neighboring base stations and compares the least pilot power strength out of the stored pilot power strengths with the measured pilot power strengths of the neighboring base stations to try handoff according to the comparing result and to reduce occurrence frequency of handoff, and thereby to reduce power consumption of the mobile station and signal processing loads of the service base stations
According to a second aspect of the invention, the above object is achieved by a handoff trial method by a mobile station comprising the steps of: a first step of measuring and storing pilot power strengths of a plurality of base stations on service; a second step of measuring and storing pilot power strengths (pilot_Bxe2x80x2) of neighboring base stations; a third step of selecting the least pilot power strength (pilot_C) out of the stored pilot power strengths and comparing the sum of the pilot_C and pilot threshold_2 set to the mobile station to try handoff with the pilot_Bxe2x80x2; a forth step of, if the pilot_Bxe2x80x2 is larger than the sum of the pilot_C and pilot threshold_2 as a result of the comparing, storing associated pilot offsets out of the pilot_Bxe2x80x2 at predetermined times and counting the stored times; a fifth step of identifying whether the counting times reaches at a predetermined times to detect a pilot signal suited for handoff condition; a sixth step of, if the counting times reaches at the predetermined times as a result of the identifying, identifying whether the stored pilot offsets are all the same; and a seventh step of, if the pilot offsets are all the same as a result of the identifying, sending a signal measurement message having the stored pilot offsets and associated pilot power strengths to the service base stations and requesting handoff therefrom.
Another object of the present invention is to provide a handoff trial method by a mobile station which measures and stores pilot power strengths of more than two base stations providing communication service to the mobile station and compares the stored pilot power strengths each other to try call-cut according to the comparing result and to reduce occurrence frequency of handoff, and thereby to reduce power consumption of the mobile station and signal processing loads of the service base stations
According to a third aspect of the invention, the above object is achieved by a handoff trial method by a mobile station comprising the steps of: a first step of measuring and storing pilot power strengths of a plurality of base stations on service; a second step of selecting the largest pilot power strength (pilot_D ) out of the stored pilot power strengths and comparing a value that subtracts pilot threshold_3 set to the mobile station to try handoff from the pilot_D with each remaining pilot power strength (pilot_E), except for the biggest pilot power strength; a third step of, if there is a pilot power strength smaller than the subtracted value as a result of the comparing, storing associated pilot offsets of the pilot power strength at predetermined times and counting the stored times; a forth step of identifying whether the counting times reaches at a predetermined times to detect a pilot signal suited for handoff condition; a fifth step of, if the counting times reaches at the predetermined times as a result of the identifying, identifying whether the stored pilot offsets are all the same; and a sixth step of, if the pilot offsets are all the same as a result of the identifying, sending a signal measurement message having the stored pilot offsets and associated pilot power strengths to the service base stations and requesting handoff for call-cut therefrom.